•Fig. 27.3 Comparisonofthemorphologyoftheepitheliumofthesmallintestineandcolon.
The lamina propria immediately below the epithelium consists largely of loose connective tissue that contains collagen and elastin fibrils (see
Fig. 27.2 ). The lamina propria is rich in several types of glands and contains lymph vessels and nodules, capillaries, and nerve fibers. The muscularis mucosae is the thin innermost layer of intestinal smooth muscle. When seen through an endoscope, the mucosa has folds and ridges that are caused by contractions of the muscularis mucosae.
The next layer is the submucosa (see
Fig. 27.2 ), which consists largely of loose connective tissue with collagen and elastin fibrils. In some regions of the GI tract, glands (invaginations or folds of the mucosa) are present in the submucosa. The larger nerve trunks, blood vessels, and lymph vessels of the intestinal wall lie in the submucosa, together with one of the plexuses of the enteric nervous system (ENS), the submucosal plexus.
The muscularis externa, or muscularis propria, typically consists of two substantial layers of smooth muscle cells: an inner circular layer and an outer longitudinal layer (see
Fig. 27.2 ). Muscle fibers in the circular muscle layer are oriented circumferentially, whereas muscle fibers in the longitudinal muscle layer are oriented along the longitudinal axis of the tube. In humans and most mammals, the circular muscle layer of the small intestine is subdivided into an inner dense circular layer that consists of smaller, more closely packed cells, and an outer circular layer. Between the circular and longitudinal layers of muscle lies the other plexus of the ENS, the myenteric plexus. Contractions of the muscularis externa mix and circulate the contents of the lumen and propel them along the GI tract.
The wall of the GI tract contains many interconnected neurons. The submucosa contains a dense network of nerve cells called the submucosal plexus (also referred to as Meissner’s plexus). The prominent myenteric plexus (Auerbach’s plexus) is located between the circular and longitudinal smooth muscle layers. These intramural plexuses constitute the ENS. The ENS helps integrate the motor and secretory activities of the GI system. If the sympathetic and parasympathetic nerves to the gut are cut, many motor and secretory activities continue because the ENS directly controls these processes.
The serosa, or adventitia, is the outermost layer of the GI tract and consists of a layer of squamous mesothelial cells (see
Fig. 27.2 ). It is part of the mesentery that lines the surface of the abdominal wall and suspends the organs within the abdominal cavity. The mesenteric membranes secrete a thin viscous fluid that helps lubricate the abdominal organs so movement of the organs can occur as the muscle layers contract and relax.
Regulatory Mechanisms in the Gastrointestinal Tract
Unlike the cardiovascular or respiratory systems, the GI tract undergoes periods of relative quiescence (intermeal period) and periods of intense activity after the intake of food (postprandial period). Consequently the GI tract has to detect and respond appropriately to food intake. In addition the macronutrient content of a meal can vary considerably, and there have to be mechanisms that can detect this and mount appropriate physiological responses. Thus the GI tract has to communicate with associated organs such as the pancreas. Finally, because the GI tract is essentially a long tube, there have to be mechanisms by which events occurring in the proximal portion of the GI tract are signaled to the more distal parts, and vice versa.
There are three principal control mechanisms involved in the regulation of GI function: endocrine, paracrine, and neurocrine (Fig. 27.4